Everybody agreed that cities as they function now cannot continue sustainably. All the evidence you've presented about cities being parasitic or "colonial" is based on the way they are now, while we were all making suggestions about how different they could be based on various ideas both modern and reflecting on past human endeavors, and would be if the constraints and demands of capitalism were absent (which is really much more than just "employee - employer relations" and would have far-reaching fundamental affects on how things are done that I'm not sure you've acknowledged), and indeed would have to be in order to continue.
Likewise with data you presented on fertilizer use in modern industrial agriculture, data which resulted from the dysfunctional system we have now and all the unimaginable waste, inefficiencies, depletion and destruction it generates. This cannot continue, but I also don't think it represents any useful baseline for us to extrapolate from because it is so thoroughly a result of capitalist practices.
>>The "end" we need to prepare for is the end of high-energy lifestyles, and that includes big cities.<<
Absolutely agree about high-energy lifestyles, but am not convinced that cities necessarily have to be that way. Others have pointed out already that cities do present opportunities for more efficient use of energy and other resources in many cases (even if those aren't being exploited now as much as they could be).
For sure we could not expect to maintain cities in their current form, but to me this is because of many other considerations around capitalist technology and lifestyles, not about simple numbers of people gathered together (up to a point, of course). Every location will have a unique capacity. I don't know if cities could continue to be as large as many are now, probably not, but I'm not convinced they couldn't or shouldn't exist at all.
And always lurking in the back of my mind was to cut to the chase about coercion. If you also do not support the use of institutionalized coercion then what are you going to do about these cities that you think should not exist? How would you disperse them in a voluntary way?